1. Field of the Invention
The present invention relates generally to an apparatus having a printed circuit board assembly, and more particularly to a cooling structure for an apparatus such as a central processing unit (CPU) of a computer system.
2. Description of the Related Art
Electrical and electric equipment such as a CPU comprises a housing, a printed circuit board assembly stored in the housing, a power supply, etc. The printed circuit board assembly includes at least one high-heat-generating board (typically, a CPU board) and at least one non-high-heat-generating board (typically, a memory board).
In this type of electrical and electric equipment, higher integration density of which has been demanded, the printed circuit board assembly has a mother board for supporting printed circuit boards. The mother board is provided with a number of board-insertion connectors. Each connector of the mother board receives a printed circuit board. The mother board functions as means for gathering the high-heat-generating and non-high-heat-generating boards. By this structure, a number of printed circuit boards are attached to the surface of the mother board in a laminated manner, and the assembly of the printed circuit boards realizes higher integration density of various electric circuits such as a CPU, a memory, an interface, etc.
In this electrical and electric equipment, the printed circuit board assembly constitutes a predetermined electric circuit. In order to make the electric circuit function according to specifications, it is necessary to provide suitable cooling means.
Conventional cooling means for the printed circuit board assembly has, for example, the following structure. An air suction unit is provided on a panel of a housing, and a fan is provided at the top of the housing. According to this structure, an air stream can be produced forcibly in the housing, and the printed circuit board assembly mounted within the housing can be cooled by the air stream. According to another example of the conventional cooling means, a fan is provided at the top portion of the housing or at the bottom portion of the housing, or at both the top and bottom portions. By this structure, the air taken in by the fan can be applied directly to the printed circuit board assembly within the housing.
The above conventional cooling means is suitable when printed circuit boards having the same heat-generation characteristics are uniformly cooled. However, it is not suitable for the structure wherein a large-sized rectangular printed circuit board is used or only a specified printed circuit board has a high integration density and high heat generation characteristic.
The drawbacks in the prior art will now be explained in detail. Generally, a printed circuit board assembly includes a mother-board. The mother board functions to gather signal lines and power supply lines of printed circuit boards. The number of signals transmitted between printed circuit boards has recently been increased in accordance with the enhancement of functions of the printed circuit boards. Similarly, the number of board-insertion connectors for connection between the printed circuit boards and mother board has increased more and more.
Each printed circuit board of the printed circuit board assembly has generally a rectangular shape, with a longer side put in contact with the mother board, as the number of signals transmitted between the printed circuit board and other printed circuit boards and mother board has increased.
On the other hand, with development of integrated circuit techniques and a demand for reduction in circuit size, there is a tendency that a greater number of circuits are integrated in a small device. In this case, a device in which many circuits are operated at high speed generates a large quantity of heat, and a printed circuit board with such a device also generates a large quantity of heat. This tendency is most conspicuous in a CPU board. Of all printed circuit boards, the CPU board generates the largest quantity of heat, and the other boards (typically, memory boards) generate less heat.
According to the structure of conventional cooling means, an air suction unit is provided on a panel of a housing and a fan is provided at a top portion of the housing; thus, an air stream is forcibly produced within the housing and the printed circuit board assembly in the housing is cooled by the air stream. Alternatively, a fan may be provided at a bottom portion of the housing and air taken in by the fan is directly applied to the printed circuit board assembly within the housing.
When such conventional cooling means is applied to the electrical and electric equipment having large rectangular printed circuit boards or printed circuit boards some of which have a high integration density and high heat generation characteristic, the cooling efficiency lowers and a local heat-accumulation phenomenon occurs.
Specifically, since the fan is situated within the housing which defines a limited space, the size (performance) of the fan capable of being mounted in the electrical and electric equipment is naturally limited. Accordingly, a part of a general printed circuit board assembly, which requires cooling particularly, and another part thereof, which does not require cooling particularly, are similarly cooled. Consequently, since the fan with a limited performance is used, the part of the printed circuit board assembly which requires cooling particularly is not sufficiently cooled.
In an air cooling system, heat generated by a printed circuit board is absorbed and carried by an air stream flowing above the printed circuit board. Thus, a large quantity of heat can be absorbed and carried from the upstream side of the printed circuit boards.
However, since the air stream is warmed, only a small quantity of heat is absorbed and carried from the downstream side of the printed circuit boards. In particular, when an air stream is let to flow in the longitudinal direction of large rectangular printed circuit boards, there is a large influence of a heat-accumulation phenomenon in the downstream side of the printed circuit boards.
Means for overcoming this drawback is the structure wherein an air stream is let to flow in the transverse direction of the rectangular printed circuit board at the longitudinal end portion thereof. However, because of the presence of the mother board, it is difficult to provide an air passage in the transverse direction, and this means is difficult to realize.
Under the circumstances, in order to reduce the influence of the heat-accumulation phenomenon in the downstream side of the printed circuit boards, a fan having a high performance is required. However, as stated above, the size (performance) of the fan is limited since it is mounted in the housing.